Selenium is one of the potentially toxic trace metals found in the coal and lignite customarily used as energy sources for electric power plants. The combustion of selenium-containing coal or lignite typically produces selenium oxide (SeO.sub.2) in the flue gas, although there is some disagreement on this point. Selenium may predominate as the free element (Se.sub.6) in flue gas at flue gas temperatures around 150.degree. C. (302.degree. F.). Whether it is present as the oxide or the free element, the removal of selenium from flue gas prior to the discharge with the stack gases produced by the power generation process is highly desirable.
Virtually all of the selenium from the combusted coal or lignite is found in some form in the combustion waste gas. Typically, electric power plant waste gases are subjected to one or more treatments to remove particulates and controlled components such as sulfur oxides and nitrogen oxides. Particulates may be removed by electrostatic precipitators, baghouses and scrubbers designed to remove solid particles from gaseous media. After particulates are removed, the flue gas is generally directed to a flue gas desulfurization system where sulfur and nitrogen oxides are removed from the flue gas so that it meets standards for the discharge of stack gases to the environment. If selenium is not removed from the flue gas prior to the scrubbing of the gas in the flue gas desulfurization process, separation of the selenium from the flue gas desulfurization solids will be required after scrubbing, which can be difficult and costly.
U.S. Pat. No. 5,192,514 to Dismukes, which is commonly owned by the assignee of the present invention, discloses a method for the removal of gaseous selenium from flue gas. This method does not humidify the flue gas, but involves injecting ammonia into flue gas produced by coal combustion at a location between an air heater and a baghouse, although it is disclosed that the ammonia may also be added to the flue gas stream in the baghouse. However, ammonia is expensive and can be somewhat dangerous to handle.
U.S. Pat. No. 3,966,889 to Kakuta et al. describes a wet scrubbing process for removing selenium from waste gas, especially that produced by a glass melting furnace. The waste gas is contacted with an absorbing solution containing an alkali metal sulfite or bisulfite. An acid treatment reduces the selenium present in solution to metallic selenium. However, in this process, the waste gas is contacted with an excess of a water-based reagent and is thus closer to a wet flue gas desulfurization process scrubbing step. In the process described in this patent, selenium removal occurs simultaneously with the sulfur dioxide removal and, thus, does not avoid the problems which accompany high selenium concentrations in the liquid streams exiting the flue gas desulfurization system.
U.S. Pat. No. 5,281,402 to Gohara et al. discloses a method of saturating and humidifying flue gas in a flue gas desulfurization process to removal gaseous contaminants such as mercury. However, this is essentially a scrubbing process which uses recycled slurry to form a continuous liquid curtain through which the flue gas is passed. As a result, sulfur oxides would be removed with the contaminants, and it would be necessary to separate the contaminants from the sulfur oxide-containing scrubbing byproducts.
U.S. Pat. No. 3,130,012 to Prater et al. discloses a process for the recovery of selenium from the gaseous effluents produced by copper smelting. In this process an anionic surface active agent in an aqueous solution is used in conjunction with a selenium coagulant to remove and recover selenium values. It is not suggested that humidification of the flue gas in conjunction with the injection of alkaline materials could be employed to remove substantially all of the vapor phase selenium from a flue gas to be subjected to a flue gas desulfurization process.
In U.S. Pat. No. 4,273,747, Rasmussen discloses the removal of mercury from waste gases produced by the incineration of domestic waste. An aqueous liquid, which may be hydrated lime, is atomized into the waste gas stream, which is stated to have a temperature of at least 200.degree. C., in a quantity higher than required to cool the waste gas stream to a temperature of 160.degree. C., but lower than the temperature required to cause complete water evaporation. The mercury is separated out in the fly ash, which is recovered as a free-flowing powder. It is not suggested that this method could be used to effect selenium removal from selenium and sulfur oxides-containing flue gas produced by the combustion of coal. Moreover, the temperature of the flue gases produced by coal combustion is typically well below the 200.degree. C. temperature required by the Rasmussen process.
U.S. Pat. No. 5,435,980 to Felsvang et al. and the paper entitled "Air Toxics Control By Spray Dryer Absorption" by Felsvang et al. describe the removal of mercury vapor in dry flue gas desulfurization systems. In the method of U.S. Pat. No. 5,435,980, an aqueous suspension of a basic absorbent is atomized into hot flue gas in a drying chamber of a drying absorption zone. The chloride content of the flue gas or absorbent is adjusted to achieve high mercury removal. In the "Air Toxics Control" publication, mercury is removed by an activated carbon injection system upstream of a spray dryer absorber. Chlorine is added as required to enhance mercury removal when the chloride content of the coal is low. The "Air Toxics Control" publication suggests that other air toxics, including selenium and boron, could also be removed in a dry flue gas desulfurization system. However, these references do not suggest that the removal of vapor phase selenium across a wet flue gas desulfurization process could be achieved by humidification of the flue gas in conjunction with the injection of an alkaline material upstream of the scrubber in a flue gas desulfurization system.
The prior art, therefore, has failed to provide an effective method for efficiently and effectively removing substantially all of the vapor phase selenium from selenium and sulfur oxide-containing flue gas produced by a coal burning power generating plant across a wet flue gas desulfurization system. A need exists for such a method.